I know not all, but Debian/Ubuntu/Fedora does, and while the intended purpose of multi-stage releases is not necessarily security but stability, it still does help up with security too. Because third parties can look and scan the dependencies while they are still not in stable.

Most of the supply chain vulnerabilities that ended up in the NPM would have been mitigated with having mandatory testing / stable branches, of course there needs to be some sort of way to skip the testing but that would be rather rare and cumbersome and audited, like it is in Linux distributions too.

Is a "AUR" now just how we name unaudited software repositories?

Just to note, if we're talking about Linux Distributions. There's also COPR in Fedora, OBS for OpenSUSE (and a bunch of other stuff, OBS is awesome), Ubuntu has PPAs. And I am sure there's many more similar solutions.